Currently marketed passenger vehicles are often equipped with a steer by wire steering device in which front wheels are mechanically decoupled from a steering wheel, and steered by a steering actuator in response to the operation of the steering wheel. In a steer by wire steering device, a steering unit having a steering actuator for turning the front wheels and a reaction force actuator for applying a steering reaction force to a steering wheel is provided. In order to allow the steering operation to be performed at the time of a failure, the steering unit and the steering wheel can be connected to each other via a clutch which is normally disengaged but can be engaged when required. See JP5840168B, for instance.
In such a steer by wire device, when a failure in the steering system is detected, the clutch is engaged so that the road wheels may be directly steered by the steering wheel. However, during the time interval between the detection of the failure and the full engagement of the clutch, the steering system may behave in an unpredictable manner. For instance, the road wheels may be steered by a road surface irregularity. In such a case, the vehicle operator may experience some discomfort. To overcome this problem, it is proposed in JP4635720B to perform a position keep control when a failure of a reaction force actuator is detected, whereby the steering angle of the road wheels is fixed by creating a braking force with the steering actuator for a time period required for verifying the failure. If the failure is verified, and the steering directions of the road wheels and the steering wheel do not agree, the position keep control is discontinued, and the steering actuator is operated according to the steering angle of the steering wheel until the clutch is engaged.
However, in the control method of JP4635720B, an impulsive torque may be transmitted to the steering wheel when the clutch is engaged, causing discomfort to the vehicle operator.